Some combustion characteristics of a fuel can be measured to predict the fuel's performance in a particular engine. For example, fuels that may be used in homogeneous charge compression ignition (HCCI) engines, including some diesel engines, are frequently analyzed to determine characteristics known as the cetane number and octane number. Such characteristics have previously been employed in an attempt to aid an engine designer in determining the proper fuel mixture for use in a particular HCCI engine.
The octane number can be an indication of a fuel's resistance to detonation. For example, a fuel having a high octane number will resist auto-ignition more than another fuel having a lower octane number. The uncontrolled auto-ignition of a fuel in an engine is undesirable because it leads to a phenomenon known as engine knock. A forceful knock is usually accompanied by rapid build up of pressure and vibration that can damage the engine.
The cetane number can be an indication of a fuel's propensity to auto-ignite. As such, the cetane number may affect an engine's ability to cold start, may affect the engine's emissions, and may affect the engine's combustion efficiency. For example, in a typical diesel engine, the fuel is ignited by hot air (e.g., heated by compression). The fuel is usually injected into this hot air just before the piston reaches top-center position, and in most designs, ignition should begin just as the piston reaches this position. If the fuel does not ignite when the piston is at the top-center position, the entire charge of fuel may become thoroughly mixed with air, thereby causing a steeper pressure rise when the fuel finally does ignite. Accordingly, a diesel engine that operates with a fuel having a lower-than-recommended cetane number may be difficult to start, may be generally more noisy, may operate roughly, and may have higher emissions.
Because the octane number and cetane number of a particular fuel may indicate opposite characteristics, it is usually the case that a higher cetane number results in a lower the octane number, and vice versa. Traditionally, an engine designer was required to test various fuel mixtures to determine individual cetane numbers or octane numbers and to subsequently select a fuel having a desirable compromise of the cetane number and the octane number for use in a particular engine. In some circumstances, these characteristics (cetane number or octane number) by themselves are not an adequate indicator of the fuel's performance in particular engines, including some HCCI engines that utilize blends of octane and cetane fuels.